The present invention relates to medical diagnostic imaging systems and methods, and in particular to improvements that adaptively reduce or eliminate the wraparound artifact, also known as the range ambiguity artifact or the multi-path artifact.
A frame of an ultrasound image is generated by many successive transmit firings. In commonly used imaging systems, the interval between firings (xe2x80x9cinterline delayxe2x80x9d) is determined offline and is fixed for all imaging situations. Transmit power is generally determined by either the system limitations such as the maximum output voltage and transducer sensitivity or by regulatory limitations such as maximum power levels allowed by the FDA.
In one prior art system, the interline delay between consecutive transmit events is calculated offline using predetermined attenuation coefficients. Generally, conservative values are chosen to ensure that the wraparound artifact is avoided for all imaging situations. Another prior art approach is to determine a minimum interline delay by clinically evaluating the wraparound artifact. Once again, a conservative value is generally chosen to ensure that the wraparound artifact is avoided in all imaging situations.
Another prior art approaches to acquire a frame of image data by firing scan lines from the center lines of the imaged region to the edge lines. The interline interval decreases from the center lines to the edge lines. For example, if a total number of lines in an image frame is 2n, the firing order can be as follows: n, n+1, nxe2x88x921, n+2, nxe2x88x922, . . . 2, 2nxe2x88x921, 1, 2n. The interline delay for the center line is set at a value that ensures that the wraparound artifact is avoided in all imaging situations. See the discussion in U.S. Pat. No. 5,438,994.
The prior art methods described above use pre-programmed interline delays. For this reason, they cannot achieve the optimum tradeoff between frame rate and wraparound artifact reduction for different imaging situations. The firing sequence of the third method described above is inflexible, and it is difficult to optimize both frame rate and wraparound artifact reduction at the same time with this method.
The preferred embodiments described below adaptively and automatically adjust one or both of the interval between transmit events (the line duration) and the transmit power as a function of the penetration depth. The penetration depth is measured by several approaches, as illustrated by the various embodiments. The frame rate (which is dependent on the line to duration) is then adaptively and automatically set at the maximum possible value for the prevailing imaging conditions, while simultaneously avoiding the wrap-around artifact generated by energy from the previous transmit line. Alternatively, even higher frame rates can be achieved if the penetration length is decreased to match the actual displayed depth. This can be done by decreasing the transmit power. The amount of power decrease can be estimated by measuring the attenuation coefficient in the prevailing imaging conditions. The measurement of the attenuation coefficient can be done by measuring the strength of the received signal as a function of depth.
The following detailed description describes four different methods for using receive signals acquired by an ultrasonic imaging system to estimate the maximum penetration depth, including cross correlating receive signals from two separate transmit events, differencing receive signals from two separate transmit events, comparing a set of receive signals with a background receive signal level, and assessing the true magnitude of the receive signals.
The foregoing paragraphs are intended by way of introduction, and are not intended to limit the scope of the following claims.